This invention relates to color cathode ray tubes of the type having a shadow mask, and especially to systems for suspending shadow masks of various types in cathode ray tubes.
A color cathode ray tube may be of the type which has a shadow mask assembly that includes a heavy frame to which is welded a dished, apertured mask. The mask-frame assembly is precisely mounted in relation to the tube faceplate by a suspension system comprising three or four leaf springs which are welded to the frame at spaced points around the periphery thereof. The springs have apertures at their distal ends which engage studs extending from the tube faceplate. The assembly is capable of being demounted from the faceplate and precisely remounted by depressing the springs to disengage or re-engage the studs.
The relatively heavy weight and costlines of this type of mask-frame assembly has led to the invention of a low cost, light weight, non-self-rigid, torsionally flexible shadow mask preferably of a one-piece frameless construction. A shadow mask of this type is disclosed in U.S. Pat. No. 4,100,451 to Palac, of common ownership herewith. Because of the lack of the frame in a mask of this type, the attachment of the springs to the mask is primarily by means of relatively stiff brackets which embrace the mask, and to which the brackets are welded. The suspension springs in turn are welded to suitable areas of the brackets. The bracket means of attachment provides for stiffening of the shadow mask due to the rigidity of the brackets. Also, the pressure exerted by the springs where the mask is in the mounted position, and when the mask is demounted and remounted, is absorbed by the brackets so that the non-self rigid mask is not distorted.
It is essential to the stability of the shadow mask that the point of attachment of the spring either to the frame of a mask-frame assembly or to a bracket attached to a frameless mask be stable; that is, the spring must not rock, twist or otherwise displace in relation to the point of attachment except for the intentional flexure required for demounting and remounting. The entire suspension system, including the point of attachment of the spring to the mask frame or bracket, must precisely fix and hold a predetermined spatial position of the mask as a whole relative to the faceplate against translational or rotational displacement, this in spite of any thermal expansion or contraction of the mask, frictional restraint during demounting and remounting of the mask, mechanical shocks or force of gravity.
The means for forming the permanent attachment at the interface of spring and bracket, normally spot welding, can exacerbate the displacement problem. Spot welding tends to "dimple" the parts at the interface, with the result that the parts are in contact only at the weld points. The relatively short distance between the weld points (as compared to the total width of the spring) constitutes a relatively narrow baseline upon which the spring can more readily rock or twist in relation to the bracket or frame.